Method for separating zirconium and hafnium tetrachlorides with the aid of a melted solvent

ABSTRACT

The invention relates to a method of separating zirconium and hafnium tetrachlorides using a solvent comprising firstly an alkaline metallic solvent comprising a salt made up of an alkali metal chloride and an acidic metal chloride A, for example a chloroaluminate or an alkaline chloroferrate, and secondly an acidic metal or metalloid chloride B of acidity that is less than that of the acidic metal chloride A. The acidic metal or metalloid chloride B is preferably selected from chlorides of Mg, Zn, and Cu. The method may be a continuous separation method by selective absorption of the tetrachloride vapors by the solvent in the substantially or totally molten state.

[0001] The present invention relates to an improvement to the method ofseparating zirconium and hafnium by extractive distillation using moltensalts.

[0002] Zirconium possesses a very small neutron capture section, andthat is why it is used in nuclear reactors. The mineral zirconnevertheless always contains hafnium, at a concentration of about 1% to3% by weight. Unlike zirconium, hafnium absorbs neutrons strongly andwill therefore greatly reduce neutron flux in a nuclear reactor. The useof zirconium in the nuclear field thus requires prior elimination ofhafnium, and a content of less than 100 parts per million (ppm) is oftenrecommended.

[0003] Hafnium and zirconium have properties that are very similar whichmakes separating them extremely difficult. Various techniques have beenproposed and used. At present, only a few methods have been accepted andapplied on an industrial scale. One such method is multiplecrystallization of potassium and zirconium fluoride, another isliquid-liquid extraction using various organic solvents, e.g. TBP in anacid medium (nitric acid), n-octylamine in an acid medium (sulfuricacid), methyl isobutyl ketone (MIBK), and finally there is the method ofchloride distillation (D. Sathiyamoorthy et al., High temperaturematerials and processes 1999, Vol. 18, No. 4, 213-226).

[0004] Conventionally, prior to performing extractive distillation andcertain liquid-liquid extraction techniques, the mineral is subjected tocarbochloration which produces the tetrachlorides ZrCl₄ and HfCl₄. Inliquid-liquid extraction, the chlorides need to be put into aqueoussolution and extraction leads to the formation of ZrO₂ and HfO₂, whichthen requires further carbochloration of the zirconium prior to movingon to the stage of recovering metallic Zr.

[0005] The method by extractive distillation uses a distillation columnhaving a plurality of trays each supporting a layer of molten salts. Thechlorides are introduced in the gaseous state. ZrCl₄ is recovered in thesolvent at the bottom of the column. HfCl₄ is entrained with the gas tothe top of the column. Various solvents have been proposed: sodiumchlorozirconate and chlorohafnate in FR-A-1 537 218; phosphorusoxichloride in U.S. Pat. No. 1,582,860; pure zinc chloride; anhydroustin chloride in U.S. Pat. No. 2,816,814; alkali metal chloride+aluminumor iron chloride in U.S. Pat. No. 2,928,722; sodium chloride in U.S.Pat. No. 3,671,186; alkaline chloroaluminate or chlorferrate in FR-A-2250 707 (U.S. Pat. No. 4,021,531), FR-A-2 543 162; sodium and potassiumchlorides U.S. Pat. No. 3,966,458; zinc chloride and lead chloride inU.S. Pat. No. 4,737,244; zinc chloride +calcium or magnesium chloride inU.S. Pat. No. 4,749,448; and lithium chloride+at least one chlorideselected from those of sodium, potassium, magnesium, and calcium in U.S.Pat. No. 4,874,475.

[0006] Those various separation techniques present their advantages andtheir drawbacks. It is mentioned above that liquid-liquid extractionrequires steps of putting chlorides into solution and a secondcarbochloration step. The solvent MIBK is volatile and highly explosive,which gives rise to problems with handling and reprocessing effluents,but nevertheless a significant fraction of worldwide Zr production isbased on that technique. The solvent TBP turns out to provide lowerperformance and to be more expensive, which explains why it has beenabandoned progressively. See A. B. V. da Silva, Jr. and P. A. Distin inCIM Bulletin 1998, Vol. 91, No. 1018, 221-224.

[0007] A drawback of extractive distillation results from the fact thatthe looked-for metal, i.e. Zr, is in the solvent which means that itmust subsequently be recovered therefrom and thus that additional stepsneed to be performed, which gives rise to non-negligible costs.According to da Silva and Distin supra, the main drawbacks of thattechnique are its poor separation factor which requires a very largenumber of stages (about 90), and the highly corrosive nature of thesolvents and the constraints associated with using vapor streams, whichalso has an impact on the materials that can be implemented. Stillaccording to those authors, the cost of an installation operating withthat method is therefore high, and that ought to constitute a brake onits use for new installations. Finally, that document concludes that thefuture lies more with liquid-liquid extraction using new aqueoussolvents.

[0008] Going against the teaching of that document, the Applicant hasset out to improve the productivity, i.e. the separation efficiency ofthe method of extractive distillation in molten salts, which improvementcan also be applied to existing distillation installations.

[0009] As a result, the Applicant has found that by using novelsolvents, the efficiency of separation is increased. It is also to beobserved that it is possible to invert the separation process andrecover a Zr-enriched gas from the top of the column. It is alsopossible to work at a temperature that is lower than the temperatureusually recommended, and even to further increase the separation factorsince it increases inversely with temperature. This gives rise toconsiderable consequences. It is possible to lighten the additionalsteps of recovery downstream from distillation. It is also possible toreduce the number of distillation stages. This also has a majorincidence on reducing the cost of operating existing installations andon the costs of installing and running new units. Other advantages arementioned below.

[0010] According to the present invention, these objects and results areachieved by a method of continuously separating zirconium and hafniumtetrachlorides by selective absorption of their vapors by a moltensolvent (substantially or totally molten) flowing as a countercurrent tosaid vapors in a distillation column, said molten solvent comprising atleast an alkaline metallic solvent of the type commonly used inextractive distillation of zirconium, i.e. preferably at least a saltmade up of an alkali metal chloride and an acidic metal chloride A, suchas AlCl₃ or FeCl₃ (which gives an alkaline chloroaluminate orchloroferrate). This “base” solvent has added thereto one or more acidicmetal or metalloid chlorides B that are weaker than those making up thecomposition of said “base” solvent, thus leading to a molten salt thatis at least ternary.

[0011] By definition, a chloride is said to be “acid” if it is capableof bonding with Cl⁻ ions. The term “weaker” when speaking of the acidityof the metal chloride B means that the element bonded to chlorine in theacidic metal (or metalloid) chloride B has lower affinity for Cl⁻ ionsthan does the element bonded to chlorine in the acidic metal chloride A.The acidity of the chloride or of a mixture of chlorides, i.e. its pCldefined by pCl=log[Cl⁻], can be determined in simple manner by measuringthe free potential of the chloride or of the mixture. It suffices tomeasure it on an aluminum wire and relative to a reference electrodemade up of an aluminum wire dipped in a KCl-saturated mixture ofAlCl₃—KCl. The pCl is obtained by the formula:

E=E ₀−(4RT/3F)ln[Cl⁻]

[0012] where:

[0013] E₀=the free potential of the chloride or the mixture ofchlorides;

[0014] R is a constant=8.314;

[0015] T=temperature in Kelvins;

[0016] F=Faraday's constant;

[0017] ln[Cl⁻]=the natural logarithm of the Cl⁻ ion concentration.

[0018] The acidic metal or metalloid chlorides B are selected inparticular from chlorides of alkaline earths (e.g. Mg), chlorides oftransition metals (e.g. Zn, Cu, Ni, Co, and possibly Fe; Zn and Cu beingthe preferred transition metals), or certain metalloids such as Pb andSn. The acidic metal chlorides B preferably have oxidation degree II.Advantageously, they also possesses a coordination number of 4 in themixture. It is preferably magnesium chloride MgCl₂ and/or zinc chlorideZnCl₂.

[0019] The present invention sets out to be applied to solvents made upof an alkali metal chloride and an acidic metal chloride (metalloidchloride or transition metal chloride) as found in the prior art, e.g.chloro-aluminates or chloroferrates, while nevertheless not beinglimited thereto. More particularly, in the “base” solvent, the alkalimetal chloride may comprise as its metal Li, Na, K, or Cs. KCl and NaClare preferred. The acidic metal chloride A and the acidic metal ormetalloid chloride B are selected so as to satisfy the above-specifiedrule of weaker acidity for B/A. Preferably, the acidic metal chloride Ais AlCl₃ and/or FeCl₃. It is preferred to have the followingcombinations AlCl₃+KCl and/or FeCl₃+NaCl as the base solvent. It ispossible to use AlCl₃ in the base solvent and an iron chloride as theacidic metal or metalloid chloride B. Under such circumstances, the ironchloride is preferably FeCl₂.

[0020] In preferred implementations of the invention, the followingapply:

[0021] base solvent:

[0022] AlCl₃ and/or FeCl₃ as the acidic metal chloride A and preferablywith KCl and/or NaCl as the alkali metal chloride; more preferablyAlCl₃+KCl and/or FeCl₃+NaCl; and

[0023] acidic metal chloride B:

[0024] MgCl₂ and/or ZnCl₂ and/or CuCl₂, preferably MgCl₂ and/or ZnCl₂.

[0025] Advantageously, the molar ratio of acidic metal chloride A (e.g.aluminum chloride and/or ferric chloride) over alkali metal chloridelies in the range 0.7 to 1.3. It preferably lies in the range 0.8 to 1.

[0026] The molar ratio of acidic metal or metalloid chloride B overacidic metal chloride A (e.g. aluminum chloride and/or ferric chloride)advantageously lies in the range 0.01 to 1.5, and preferably in therange 0.1 to 0.3.

[0027] The method may be conducted in particular at a temperature lyingin the range about 250° C. to about 550° C. Nevertheless, it is moreefficient to work in the lower portion of the range. Separationefficiency increases inversely with temperature. According to anadvantageous feature of the invention, the temperature lies in the rangeabout 250° C. to about 350° C. The lower limit of the temperature rangeis selected so that the solvent is substantially molten, preferablycompletely molten, and in some cases, e.g. for certain solvents, it mayeven be less than 250° C. These low temperatures confer two considerableadvantages on the method. They make it possible to reduce energyexpenditure to a great extent, which expenditure is already reducedbecause it is possible to reduce the number of trays for givenseparation efficiency compared with the prior art. Equipment corrosionis also greatly reduced, and this is extremely favorable from the pointof view of the cost of the installation and the cost of maintenance.

[0028] Distillation is generally performed at atmospheric pressure.

[0029] The present invention also provides use of a solvent comprisingat least one base alkaline metallic solvent as described herein and atleast one acidic metal solvent B as described herein, preferablysubstantially or totally molten, for the purpose of separating zirconiumand hafnium tetrachlorides. This use is not limited to the molten saltdistillation described above, but can cover any other method enablingzirconium and hafnium tetrachlorides to be separated. Zirconium andhafnium tetrachloride vapor is put into contact with the solvent of theinvention, and the tetrachlorides are separated because of the differentaffinities of the solvent for the tetrachlorides.

[0030] According to an advantageous feature, the method and the use ofthe invention seek to enrich the vapor at the top of the column inzirconium tetrachloride and thus to recover the gas from the top of thecolumn.

[0031] The invention is described below in greater detail with referenceto embodiments taken as non-limiting examples, and with reference to theaccompanying drawing, in which:

[0032]FIG. 1 is a diagram of a separator installation enabling themethod of the invention to be implemented; and

[0033]FIG. 2 is a diagram of a laboratory device enabling the operationof the method of the invention to be investigated.

EXAMPLE 1

[0034] An example of an installation enabling the method of theinvention to be implemented.

[0035] The installation comprises means 1 for feeding a distillationcolumn 2 with raw zirconium tetrachloride vapor that also containshafnium tetrachloride. A pump 3 circulates the extraction solvent of theinvention downwards along the distillation column as a countercurrent tothe flow of zirconium and hafnium tetrachloride vapors. The solventtravels in a closed circuit via the condenser absorber 4, the column 2,the boiler 5, the stripping column 6, and the tank 7, from which it isreturned by the pump 3 to the condenser absorber 4.

[0036] By way of example, the solvent may be a mixture of AlCl₃/KCl andof MgCl₂, having the following molar ratios:

[0037] 0.7<Al/K<1.3 preferably 0.8<Al/K<1

[0038] 0.01<Mg/Al<1.5 preferably 0.1<Mg/Al<0.3

[0039] The vapors enriched in zirconium tetrachloride on rising up thecolumn 2 by exchange with the solvent flowing as a countercurrent leavethe column 2 from the top thereof, and then passes through the condenserabsorber 4 in which the vapors saturate the solvent. A fraction of thevapors that is not retained by the solvent separates from the condenserabsorber 4 and condenses in the condenser 8 which is connected to theatmosphere via a vent 9, and has drawing-off means 10 for drawing-offthe condensate that is enriched in zirconium tetrachloride.

[0040] At the bottom of the column 2, the boiler 5 of temperatureadjusted to lie in the range 250° C. to 550° C., and preferably in therange 250° C. to 350° C. receives the hafnium tetrachloride solution inthe solvent which is depleted in zirconium tetrachloride as it travelsdown the column 2. This solution passes from the boiler 5 into thestripping column 6 via a flow rate control valve 11 which is controlledso as to maintain a substantially constant level of solvent at thebottom of the column 2. In the stripping column 6, the hafniumtetrachloride depleted in zirconium tetrachloride is extracted from thesolvent by means of a flow of inert gas, such as nitrogen, flowing upthe column at a countercurrent to the solvent, and entraining at the topthereof the hafnium tetrachloride vapors which are thus extracted fromthe solvent. These vapors containing a residue of AlCl₃ are entrained bythe inert gas flow through a device 12 for eliminating aluminum, e.g. asdescribed in FR-A-2 543 162. At the outlet from the device 12, thehafnium tetrachloride vapors are entrained into the condenser 13 and thepurified hafnium tetrachloride is drawn off at 14. The inert gas isentrained around a closed circuit by the booster pump 15, losses beingcompensated by introducing inert gas at 16 and any excess pressure beinglimited by a valve 17. The inert gas may be nitrogen or other gas thatdoes not react with the compounds or materials used. Also, instead ofcausing the inert gas to circulate, it is possible to encourage hafniumtetrachloride vapor to be given off in the column 6 and to pass throughthe device 12 by reducing the pressure inside the condenser 13 by meansof a vacuum pump.

[0041] The use of a solvent in accordance with the invention in such aninstallation makes it possible to extract zirconium tetrachloride in thegaseous state from the top of the column, which gas can be recovereddirectly without the usual steps of stripping and eliminating aluminumas are required when the zirconium tetrachloride is entrained by thesolvent as in the prior art. This installation thus enables thezirconium tetrachloride to be purified to as great as extent as desiredwhile using a smaller number of trays than in the prior art. Theresidual hafnium tetrachloride content depends on the number of trays inthe column and on the settings of operating conditions. In practice, itis thus possible to use a column that has fewer trays. In addition, theoptimum temperature range is lower than in the prior art, thus making itpossible to limit problems of the equipment corroding and to greatlyreduce the amount of energy that is expended.

[0042] The installation also serves to recover hafnium tetrachloridewhich can be made use of in turn.

EXAMPLE 2

[0043] This example compares separation efficiency as a function of thecomposition of the molten solvent. Two solvents of the invention arecompared with each other and also with prior art solvents from whichthey differ by the additional presence of the acidic chloride. Thedevice used for comparison purposes, as described below, issubstantially equivalent to one tray in the column.

[0044] All of the solvents used in this example are based on an alkalinechloroaluminate formed of aluminum chloride AlCl₃ and potassium chlorideKCl in the ratios specified in the table below.

[0045] Solvent 1 does not have any acidic chloride B, unlike solvents 2,3, and 4 in accordance with the invention. In this case, the acidicchloride B is magnesium chloride MgCl₂ and the molar ratios Mg/Al aregiven in the table below.

[0046] The mixtures of solvent and zirconium tetrachloride+hafniumtetrachloride were prepared in a glove box. They were then placed in aboat 18 inserted into a long silica reactor 19 (see FIG. 2). Theassembly was placed under a flow of argon (inlet at 20 outlet at 21) andintroduced into a tubular oven 22 previously raised to the selectedtemperature. A portion of the reactor 19 was kept outside the oven 22 toconstitute a cold point symbolized at 23 where vapors condensed. Afterone hour at high temperature, the condensate and the residual remainingin the boat 18 were recovered and analyzed by inductively coupled plasmaemission spectroscopy (ICP). Zr/Hf Mg/ initial Zr/Hf condensate Enrich-Al/K Al pCl (a) (b) ment¹ solvent 1 1.04 0 ≈4.8 20 T = 350° C. 17 1/1.18solvent 2 0.869 0.21 2.2 < pCl 20 T = 250° C. 45 2.25 <2.8 T = 340° 321.60 T = 350° 25 1.25 solvent 3 0.872 0.21 2.6 < pCl 20 T = 350° C. 391.95 <3.5 solvent 4 1.07 0.21 3.1 < pCl 20 T = 250° C. 29 1.45 <4 T =340° C. 27 1.35 T = 350° C. 24 1.20

[0047] The results in the Zr/Hf column for the condensate show increasedseparation efficiency when using solvents of the invention compared withprior art solvents that do not contain acidic chloride other than thosein the “base” solvent.

[0048] The results of the table also show how separation is invertedusing the solvents of the invention. It can be seen that separationefficiency increases with decreasing temperature.

[0049] Separation efficiency is at its maximum at 250° C. for a solventof type 2. A tray enables the zirconium tetrachloride rich phase to beenriched by 2.25 times, whereas without magnesium chloride, the degreeof enrichment is only 1.18 times. Fewer trays are therefore required toreduce the hafnium content of the raw zirconium tetrachloride from 3%for example (mean content in zircon mineral) to 100 ppm or even less (aconcentration that is generally accepted in the nuclear industry). Sincethe solvent 2 is not entirely molten at 250° C., it is possible tooperate and preferable to operate at a temperature that is slightlyhigher, e.g. in the range 300° C. to 350° C. Separation continues to behighly efficient.

[0050] It should naturally be understood that the invention defined bythe accompanying claims is not limited to the particular implementationsdescribed in the description above, but covers variants that do not gobeyond the ambit or the spirit of the present invention.

1/ A method of continuously separating tetrachlorides of zirconium andhafnium by selective absorption of their vapors by means of a solventthat is substantially or totally molten and flowing as a countercurrentto said vapors in a distillation column, the method being characterizedin that the molten solvent comprises firstly an alkaline metallicsolvent comprising a salt made up of an alkali metal chloride and anacidic metal chloride A, and secondly an acidic metal or metalloidchloride B of acidity that is lower than that of the acidic metalchloride A. 2/ A method according to claim 1, characterized in that themolten solvent comprises an alkaline chloroaluminate or chloroferrate ora mixture thereof, and an acidic metal or metalloid chloride B. 3/ Amethod according to claim 1 or claim 2, characterized in that the acidicmetal or metalloid chloride B comprises a chloride of an alkaline earthand/or of a transition metal and/or of a metalloid. 4/ A methodaccording to claim 3, characterized in that the acidic metal ormetalloid chloride B has a degree of oxidation equal to 2 and preferablya coordination number of
 4. 5/ A method according to claim 3 or claim 4,characterized in that the metal is selected from the group consisting inMg, Zn, Cu, Ni, Co, Fe, Pb, Sn, preferably Mg, Zn, Cu. 6/ A methodaccording to claim 3, characterized in that the acidic metal ormetalloid chloride B is a magnesium chloride and/or a zinc chloride. 7/A method according to any one of claims 1 to 6, characterized in thatthe alkali metal chloride in the alkaline solvent comprises Li, Na, K,or Cs as its metal, and preferably K or Na. 8/ A method according to anyone of claims 1 to 6, characterized in that the acidic metal chloride Ain the alkaline solvent comprises AlCl₃ and/or FeCl₃. 9/ A methodaccording to any one of claims 1 to 6, characterized in that thealkaline solvent comprises AlCl₃+KCl and/or FeCl₃+NaCl. 10/ A methodaccording to any one of claims 1 to 9, characterized in that, in thealkaline solvent, the molar ratio of acidic metal chloride A over alkalimetal chloride lies in the range 0.7 to 1.3, and preferably in the range0.8 to
 1. 11/ A method according to any one of claims 1 to 10,characterized in that the molar ratio of acidic metal or metalloidchloride B over acidic metal chloride A lies in the range 0.01 to 1.5,and preferably in the range 0.1 to 0.3. 12/ A method according to anyone of claims 1 to 11, characterized in that the method is conducted ata temperature lying in the range about 250° C. to about 550° C., andpreferably in the range about 250° C. to about 350° C. 13/ The use of asolvent as described in any one of claims 1 to 11, for separatingzirconium and hafnium tetrachlorides. 14/ A use according to claim 13,characterized in that the solvent is used partially, substantially, ortotally molten.